I've been thinking about the use of smoke as an industrial pollutant produced every time a smelter, forge, kiln, ect completes a fuel-based task. The smoke will originate at the impassable tile at the top of furnaces or left of forges.
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It will instantly attempt to rise if possible, and failing that, will spread out across the ceiling of the fortress. Smoke would not immediately dissipate; it would remain in play for quite a while and possibly begin to "reverse flood" your fortress. To prevent dwarves from becoming unhappy with a smoke-filled cavern (and possibly a fire risk as smoke and cinders reach flammable booze and fuel stockpiles) players would build their industrial sites in rooms with vaulted ceilings (two Z levels tall), add chimneys leading to the surface so smoke can escape, and wisely trap said chimneys to prevent monsters from coming in. I think this will add a new element of planning for players to enjoy, as well as be a challenge that really requires mastery of the Z axis to overcome.
Before I post it as a suggestion proper, however, I'd like to hear discussion on the subject. How does smoke currently perform in game? Will extensive changes to smoke be required for this feature, or will its generation at workshops be the only work required? Do you think this is meaningless complexity, or a novel challenge for underground fortress-planners?